1. Field of the Invention
The present invention relates to vacuum engineering, and more specifically, to cryogenic sorption pumps, and can be used to produce superclean and oil-free vacuum within a pressure range of 10.sup.2 to 10.sup.-7 Pa while evacuating any gases expecting helium and including corrosive ones from chambers of various designations, measuring from 0.01 to several hundred cubic meters in volume.
2. Description of the Related Art
There is known a cryogenic pump (SU, A, 1333833) comprising a pumping element consisting of a circular vessel containing liquid nitrogen, a porous screen arranged coaxially with the vessel within a space encompased by its inner side surface, and a sorbent located within the gap between the inner side surface of the vessel and the porous screen.
This pump is disadvantageous in that at the liquid nitrogen temperature the sorbent has a low sorption capacity at low equilibrium pressures (below 10.sup.-3 -10.sup.-4 Pa) of adsorbable gases. As a result, this type of pump is incapable of providing limiting pressures of below 10.sup.-3 Pa even after a short-time gas load. To increase the sorption capacity of the pump, the sorbent may be cooled be means of solid nitrogen down to 55-50 K, but the sorbent cannot be maintained at these temperatures for a long time because of high natural heat input to the nitrogen-containing vessel, the nitrogen contents rapidly warming up after evacuation of nitrogen vapors is discontinued. The operation of this pump is hampered by the need for frequenctly charging the vessel with liquid nitrogen and repeatly evacuating nitrogen vapors.
Another prior-art cryogenic sortion pump (M. P. Larin, Kondensatsionno-adsorbtsionnaya i sorbtsionnaya otkachka pri temperaturakh tverdogo azota, Zhurnal tekhnicheskoy fiziki, 1988 , vol. 58, No. 10, October, Nauka Publisher (Leningrad Branch), pp. 2026-2039) comprises a housing complete with a cover fitted with an inlet nozzle for connection of the space to be evacuated and, arranged in the housing, a pumping element and cooled radiation screen encompassing the pumping element. The pumping element has the form of a circular vessel designed to contain cryogenic agent and perforated heat-concudctor and porous-screen shells installed in the space defined by the inner wall of the vessel and arranged coaxially therewith. The bottom of the vessel, the heat conductor shells, and the porous screen shells are welded to a heat conductor disc to provide thermal contact between the vessel and the heat conductor shells. Two porous screen shells are arranged on both sides of the vessel walls, and the remaining ones, on both sides of the heat conductor shells, with the annular spaces between the vessel walls and the porous screen shells, as well as the annular spaces between the heat conductor shells and the porous screen shells adjacent thereto, being filled with a sorbent material. Said sapces are covered over on top with rings. The annular spaces between the adjacent porous screen shells communicate with the inlet nozzle of the pump. The cryogenic agent vessel has a circular cover with two tubes to fill cryogenic agent into the vessel and remove cryogenic agent vapors therefrom. Said tubes have their top ends secured in the housing cover.
Owing to the incorporation of a liquid nitrogen-cooled radiation screen, the heat input from the housing to the pumping element is considerably reduced in this pump.
From the standpoint of increasing the sorption capacity of the pump, which is one of the main pumping characteristics, it is desirable that for a given pump size the sorbent should occupy the maximum possible volume while for higher pumping speeds the sorbent and the porous screens should have the maximum possible surface area. In the pump under discussion, the sorbent-filled spaces are enclosed in the pumping element vessel, with the exception of the outer space adjacent to the outer side surface of the vessel. In other words, the cryogenic agent vessel occupies a sufficiently large part of the pumping element volume, which does not participate directly in the pumping process while it could have been occupied by sorbent and porous screens. As for the outer sorbent-containing space surrounding the vessel, its performance is inefficient because of the low conductivity of the gap between said space and the radiation screen. It is for the above reasons that the sorption capacity and the pumping speed of said cryogenic sorption pump are not sufficiently high.